Shaquille O'Neal is weighing in on the Giannis Antetokounmpo vs. Pacers game ball debate -- and he's making it clear, he's siding with the Greek Freak in the tiff.

The beef, of course, began late Wednesday night -- after Indiana players felt following their 140-126 loss to the Bucks ... the rookie who scored his first-ever points during the contest, Oscar Tshiebwe, deserved the game ball more than Antetokounmpo, who dropped a franchise-record 64.

Obviously feeling differently than Indiana's players, Antetokounmpo went to the Pacers' locker room area to retrieve it -- but, chaos then proceeded to break out.

When Shaq joined Charles and Babcock on Thursday's "TMZ Live" and spoke about the commotion ... he said Giannis had every right to be upset with the situation -- believing the two-time MVP should have been the one who ultimately walked away from the arena with the memento.

"By the letter of the law," Shaq said, "if you're playing at home, you should get the ball if something spectacular happens."

Giannis did end up with a game ball -- though he told media members afterward he wasn't sure if it was the basketball he used to record his historic night.

Mark Cuban, meanwhile, told TMZ Sports earlier Thursday that if the game had happened in his home arena, he believed whichever team had the rookie deserved it most.

"That's a big deal that only happens once," the Mavericks owner said.